Heat treatment of high-speed steel



Patented Dec. 30, 19 41 HEAT TREATMENT OF HIGH-SPEED STEEL mm P. Miller and Walter A. Meadville, Pa., assignors to McCrosky Tool Corporation, Meadville, Pa... a corporation of Pennsylvania No Drawing. Application June 16, 1939,

Serial No. 219,580

. V 2 Claims.

The present invention relates to the treatment of high speed steel, and more particularly to the conditioning of tools fabricated from high speed steel alloys. 1

It is the object" of the present invention to simplify the heat treatment of high speed steel tools in order to realize manufacturing economies, while at the same time obtaining tools having higher cutting efficiency as well as secondary hardness than those attainable with the treatments of the prior art.

It is a further object of the present invention to combine the quenching and drawing or tempering steps in the heat treatment of high speed steel tools in order to obtain the requisite characteristics of hardness without diminishingthe toughness or impact value of the steel. A still further object is to avoid the danger of minute fractures in the molecular structure of the steel due to sudden and wide fluctuations in temperature. "In accordance with the present invention decarburization and soft skin" are effectively avoided.

Other objects, purposes and advantages of the invention will appear from the more detailed description of the invention following hereinafter. The present invention is applicable principally to that class of alloy steels which are designated as high speed steels which are characterized by their capability of retention of their hardness and toughness at a red heat which renders such steels useful in the special application of tools hardness in the steel.

ture or to a warm condition between 200 and 300? before being drawn. The purpose and importance of the drawing or tempering operation are well known to those versed in the art. It refines "and toughens the structure of .the steel, removes strains caused by the high heat and subsequent quench, and it produces a secondary The drawing is done in a hot air furnace or in a molten bath. The drawing temperature varies from 900 to 1200' and the time from one to. four hours, depending on the analysis of the steel, the size of the tool, and

the purpose for which the tool is intended.

*The operations and treatments outlined above are expensive, time-consuming and require a substantial amount of equipment. Unless each of the operations is performed in a critically careful manner, the possibility exists of a whole batch of tools being spoiled with the attendant loss of the labor and material which had gone into the fabrication of the tool to that stage. It is the purpose of the present invention to simplify the prowhere at times the cutting edge is raised and maintained at a red heat by frictional resistance resulting from cutting metals at high speeds. Such alloy steels are well known in the art, which may contain in various proportions and combinations, tungsten, chromium, vanadium, cobalt and molybdenum.

Heretofore, the heat treatment of such steels or tools fabricated therefrom usually involved a preheating of the steel at about 1500? to 1700 F. with or without a preliminary warming. Following this preheating, the alloys were heated. to a high temperature ranging from 2200 to 2450 F. depending upon the grade of steel undergoing treatment and the purpose for which the tool was intended. After a uniform heat, dependent in time upon the size and shape of the material being heated, the steel was quenched directly in a quenching medium of various types, usually in warm oil and sometimes in an air blast. In a few cases high speed steel tools are quenched in a molten salt or lead bath at varying temperatures, but in all cases after the quench the tools are allowed to cool down either to room temperacedure outlinedabove, reduce the cost of the manufacturing steps and at the same time to produce a superior product, which has uniformly a suitable high-speed steel is executed at a temperature range approximating 1500-1650 F. with or without a preliminary warming. Then the usual rapid heating to a high temperature of the tools from 2200-2450 F. is executed. The treatment following the high heating step constitutes the essence of the present invention and operates as a combination of. the quenching and drawing operations which are so correlated in the same bath to produce improved tools at a lower cost. The tools, which may be disposed in a basket, are quenched in a fused salt bath at a temperature of about 900-l200 F. and are permitted to remain in this bath for the drawing operation. It is preferable to adJust the fused salt bath at a slightly lower temperature than the intended drawing temperature at the time of the introduction of the tools into the bath and thereafter the temperature of the bath can be raised to the desired drawing temperature, which is maintained during the drawing period of from one to four F, and the basket of tools may be left there in for the drawing period A raising of the temperature of 40 or 50 following the introduction of the tools in the bath operates to give a refining period, the tools are removed and cooled to room temperature and are then washed in boiling water, or in acetic acid solution, or otherwise cieaned of any salts that maycling to the tools. The tools are then ready for the finishing grinding operation.

in lieu of a single period of immersion of the tools in the combined quenching and drawing salt bath, several successive short drawings instead of one long draw may be executed by raising the basket of tools into the atmosphere for a few minutes until the tools cool down toapproximately 700 to 900 F. whereupon they may be submerged again into the bath. This operation may be repeated several times and four of suchsuccessive handlings have been found advantageous without involving an undue amount of labor, Thus four drawing periods of one hour each may be executed in lieu of one long drawing period or any number of draws greater than one may be employed.

The salts used for the combined quenching and tempering are of a chloride carbonate cyanide composition the function of which are to have and maintain the proper temperature range without decomposing, to be neutral both to the container and the materials undergoing treatment, to be free from fuming at the operating temperature, and tohave high solubility to permit a clean washing at the termination of the treatment.

The fused salt bathof proper composition efiects no decarbcnization of the tools despite theirlong period of submersion therein.

"the treatments 'outlinedabove are characterized by the fact that at no time are the tools cooled to room temperature which necessitates a complete reheating treatment for the final conclitioning of the tools. The heating treatments accordance with the invention are executed expeditiously and continuously from the initial preheating operation to the final cleaning operation. The fact that the quenching and the drawing operations are performed in the same 1100 F., in a plurality of stages of extended pethe initiation of the quenching operation. If

the tools are raised from the saltbath into the air during the drawing period, as previously described, a sufiicient coating of molten salt will cling to the tools to insulate them entirely from the air during that time and thus prevent'any decarburization.

The high speed steel tools produced in accordance with the present invention have a Rockwell C hardness which averages from one to one and one-half points higher than tools produced by the usual methods of the prior art. Hardnesses of 0 65 and C-67 by the use of a quenching temperature of 980 to l000 and a drawing temperature of 1040 to 1050 are'obtained. These advantages result from the novel treatments outlined above.

Furthermore, the treatments in accordance with the present invention exclude any possibili ties of objectionable strains being setup in the tools which may arise from the practices of the 0 prior art when the tools are plunged suddenly into the drawing bath of approximately l000 to 1100 F. from room temperature. The sudden expansion of th tool creates strains which show up later in checks or cracks in th tool. With the use of a combined quenching and, drawing operation the temperature of the tools never drops below 800 to 500 F. until the conclusion of the heat treatment, and thus sudden shocks to the metal with the attendant undesirable effects are avoided.

What we claim is:

1. The method of heat treating highsp'eed steel which comprises heating the steel to ahigh temperature, in the range of from 2200 to 2450 F., quenching and drawing the steel in a fused bath at a lower temperature approximating 1000 to riods of submersion in the fused bath totaling a plurality of hours and interrupted by short intervals of withdrawal therefrom for cooling to a v temperature in the range of approximately 700 bath, simplifies the actual operative steps, and

in addition reduces the capital investment and the consequent operating costs, since one furnace performs the function of two. Of course, it is perfectly practicable for more than one furnace to be utilized, particularly in the drawing operations which are'composed of a series of successive short draws, in which case, the initial quench may be executed ,in the fused salt bath nearest to the high heat furnace and following the first withdrawal after a drawing period of I to develop the proper heat treating conditions.

to 900 F., and finally withdrawing the steel from the fused bath for cooling preparatory to the a higher degree for drawing the'tools at a temperature approximating 1050 F. by a plurality of extended submersions in the fused bath totaling a plurality of hours interrupted by short in ,tervals of withdr'awa] therefrom for cooling in the range'of approximately 700 to 900 F., and a withdrawing the tools from the fused bath for final cooling.

FRANK P. MILLER. WALTER A. REESE. 

